The present invention relates to a magnetoresistive device and more particularly to an improved ferromagnetic tunnel junction device and a method of forming the same.
The ferromagnetic tunnel junction device has a tunnel barrier layer which comprises a thin oxide layer of about a few nanometers in thickness which is sandwiched between two ferromagnetic layers During application of a constant current between the two ferromagnetic layers, an external magnetic field is applied in a plane of the two ferromagnetic layers, whereby a magnetoresistance effect appears where a resistance is varied in accordance with a relative angle between magnetization directions of the two ferromagnetic layers. If the magnetization directions of the two ferromagnetic layers are parallel to each other, then the resistance value of the ferromagnetic tunnel junction device is minimum. If, however, the magnetization directions of the two ferromagnetic layers are anti-parallel to each other, then the resistance value of the ferromagnetic tunnel junction device is maximum. If a difference in coercive force is given to the two ferromagnetic layers, then it is possible to realize the parallel and anti-parallel state of the magnetization directions of the two ferromagnetic layers in accordance with the intensity of the externally applied magnetic field, for which reason it is possible to detect the magnetic field by detecting the variation in resistance of the ferromagnetic tunnel junction device.
In recent years, a ferromagnetic tunnel junction device could be obtained which exhibits about 20% rate of variation in magnetoresistance wherein a surface oxide film of Al is used as a tunnel barrier layer. For this reason, such the ferromagnetic tunnel junction device has become attractive for applications to the magnetic head and magnetic memory device. A giant magnetoresistive device is, for example, disclosed in Journal of Applied Physics, Vol. 79, pp. 4724-4729, 1996.
A method of forming the above conventional giant magnetoresistive device will be described with reference to FIGS. 1A through 1D.
With reference to FIG. 1A, by use of an evaporation mask, a first CoFe ferromagnetic layer 82 is selectively evaporated in a vacuum on a glass substrate 81.
With reference to FIG. 1B, the evaporation mask is replaced by a fresh evaporation mask before an Al layer 83 having a thickness in the range of 1.2 to 2.0 nanometers is selectively evaporated on the first CoFe ferromagnetic layer 82 and on the glass substrate 81 but only in an adjacent region to the first CoFe ferromagnetic layer 82.
With reference to FIG. 1C, a surface of the Al layer 83 is subjected to an oxygen glow discharge to oxidize the Al layer 83 into an alumina tunnel barrier layer 84.
With reference to FIG. 1D, a second Co ferromagnetic layer 85 is deposited over the alumina tunnel barrier layer 84 and the glass substrate 81, thereby to form a ferromagnetic tunnel junction device which exhibits a 18% maximum rate of variation in magnetoresistance.
The other ferromagnetic tunnel junction devices are disclosed, for example, in Japanese laid-open patent publications Nos. 5-63254, 6-244477, 8-70148, 8-70149 and 8-316548. Further, in Japan Applied Magnetic Vol. 21, pp. 493-496, 1997, it is disclosed that an Al layer is exposed to an atmosphere for oxidation to form an alumina tunnel barrier layer.
The conventional method of forming the ferromagnetic tunnel junction device has the following problems.
In order to apply the ferromagnetic tunnel junction device to the magnetic head or magnetic memory device, it is necessary to reduce an influence to sensitivity by thermal noises, for which reason a relatively low resistance is required even in a practical size. It is however difficult for the conventional method to realize those requirements.
Further, in order to apply the ferromagnetic tunnel junction device to the magnetic head responsible for the high density recording, a high signal output voltage is important. It is however, difficult for the prior art to obtain a sufficiently high current density without deterioration of any characteristic or property of the ferromagnetic tunnel junction device.
Furthermore, there are large variations in device characteristics and properties in a wafer or between lots. It is also difficult for the prior art to obtain a required high yield of manufacturing of the ferromagnetic tunnel junction device.
Those problems are considered due to the conventional method of forming the tunnel barrier layer. In the method of using the oxygen glow discharge, activated oxygen in states of ions and radicals is utilized for oxidation of the electrically conductive layer. For this reason, there is raised a problem with a difficulty in control of thickness of a thin oxide layer or control of a resistance of the device. A further problem is raised with a deterioration in junction quality due to contamination of the tunnel barrier layer by activated impurity gases.
In the meantime, the other method using the natural oxidation by exposing the conductive layer to the atmosphere has a problem with formation of pin holes in the tunnel barrier layer due to dusts in the atmosphere. Further, a problem is raised with a deterioration in junction quality due to contamination of the tunnel barrier layer by moisture, carbon dioxide, and nitrogen oxide. As used herein, "natural oxidation" refers to a technique in which the surface to be oxidized is exposed to an atmosphere that includes oxygen and the oxidation of the surface occurs without plasma or glowing oxidation techniques.
In the above circumstances, it had been required to develop a novel method of forming a tunnel barrier layer sandwiched between first and second ferromagnetic layers free from the above problems.